1. Field of the Invention
The present invention relates to a building board, and method and apparatus for coating a building board. In particular, it relates to a building board having concavities such as grooves to which a shade-coating is effectively applied to obtain an aesthetically enhanced building board.
2. Description of the Prior Art
A building board having grooves in its surface is widely used, and the surface is generally coated to obtain an aesthetically enhanced building board. In a conventional coating method, a spray coating of a joint portion color is applied to a surface of a substrate such as a cement board in such a manner that grooves are also coated, and then a coating of an appropriate color is applied to the surface by a roll coater method, a curtain flow coater method or the like.
FIG. 37 shows a cross-section of a conventional building board having its surface subjected to surface coating by a roll coater method. The building board (a) has grooves 5 in its surface. In the surface coating, a coating (a1) of a joint portion color is first formed over the whole surface including the grooves 5 by a spray coating method, and then an appropriate coating (a2) is formed on convexities (surface plateau) by a roll coater method. In a roll coater method, if the convexities are even as shown in FIG. 37, a uniform coating is formed. However, with respect to a building board having a surface with considerable irregularities, a non-uniform coating is formed. Accordingly, appropriate surface coating cannot be effected. Further, it is impossible by a roll coater method to aptly a specific coating to the grooves 5. The grooves 5 are coated merely with the spray coating (a1) of a joint color, and thus all surfaces defining the grooves 5 have substantially the same color depth. It is, therefore, impossible to impart delicate impression of shading to the grooves 5. If a sponge roll is used, a uniform coating can be applied to a surface with irregularities to some extent. Practically, however, this is possible up to up-and-down roughness of about 3 mm. Further, a desired specific coating cannot be applied to the grooves 5.
In a curtain flow coater method or the like, a coating preferable to some extent can be applied even to an up-and-down surface with considerable roughness of a building board, and the coating can be applied also to the grooves continuously. However, the coating has uniform thickness and color depth. Accordingly, a coating with shading cannot be applied to the grooves. Further, although a coating suitable to some extent can be applied to rectilinear grooves, a uniform coating cannot be applied to curved grooves or grooves with a varying groove width. Moreover, coating by a curtain flow coater method has a lower limit in a width of a groove to which it can be applied. Accordingly, if grooves are narrow to a certain extent, the coating is disadvantageously difficult.
Due to the above problems, conventional surface-coated building boards unavoidably give a viewer strong impression of flatness as a whole and, in particular, have poor impression of shading at concavities such as grooves. It is, thereby, impossible to obtain a highly aesthetic building board. Furthermore, both of the above-described methods are directed to application of a coating to a building board having a relatively surface geometry, for example, a building board having a flat surface formed with several linear grooves, and accordingly, incapable of applying a desired coating to a building board having a surface with complicated irregularities or a complicated patten. In recent years, it has been possible to easily prepare a building board having a complicated surface geometry by an extrusion molding method or a cast molding method which is highly formative, and it has been demanded to apply a desired coating to a surface having such a complicated irregularities, in particular, concavities in the surface. However, an aesthetically satisfactory surface coating has not yet been obtained.